FIG. 1 illustrates a plan view of the optical components of a typical Transmitter Optical Subassembly (TOSA) 2. In the typical TOSA, an aspheric lens 3 receives light generated by a laser diode 4 and focuses the light into the end 6 of an optical fiber stub 5. The optical fiber stub 5 is a short length of single-mode (SM) fiber that is secured within a ferule (not shown) and which has been polished on the end 6 by a polishing process.
FIG. 2 illustrates a plan view of the fiber stub 5 shown in FIG. 1 having an end 7 that is interfaced with an end 14 of a multi-mode (MM) fiber 11 of a MM patch cord (not shown). The ends 7 and 14 of the stub 5 and MM fiber 11, respectively, are typically interfaced to each other by a simplex connector (not shown). The SM fiber stub 5 has a cylindrically-shaped core 8 and a cladding layer 9 that surrounds the core 8. Likewise, the MM fiber 11 has a cylindrically-shaped core 12 and a cladding layer 13 that surrounds the core 12. It can be seen that the diameter of the core 12 of the MM fiber 11 is significantly greater than that of the core 8 of the SM fiber stub 5. Light propagating through the core 8 of the SM fiber stub 5 is coupled into the end 14 of the MM fiber 11 and propagates along the core 12 of the MM fiber 11, which is an intended result. In addition, it can also be seen in FIG. 2 that light propagating in the cladding layer 9 of the stub 5 also is coupled into the core 12 of the MM fiber 11, which is an unintended and undesirable result.
The light propagating in the cladding layer 9 of the fiber stub 5 is caused by improper optical coupling conditions (e.g., transversal misalignment, de-focalization, etc.) or aberrations, which result in modal dispersion of the light generated by the laser diode 4 and coupled into the end 6 of the fiber stub 5. This modal dispersion in the stub 5 leads to multiple modes being coupled from the end 7 of the stub 5 into the end 14 of the MM fiber 11, and thus into the core 12 of the MM fiber 11. Modal dispersion is undesirable, and different approaches are used to prevent or lessen it. The use of the aspheric lens 3 helps confine the light launched from the laser diode 4 to the core 8 of the stub 5 such that the light excites the central, or fundamental, mode of the SM fiber stub 5. Generally, a perfect peak alignment is needed to prevent or inhibit transversal misplacement and de-focalization of the light launched from the laser diode 4. An alternative to using an aspheric lens for this purpose is to use a ball lens with a pin hole to confine the light such that it is only launched into the core 8 of the stub 5.
The lens of a TOSA of the type shown in FIG. 1 is generally the second-most expensive component of the TOSA. The laser diode typically is the most expensive component of the TOSA. Aspheric lenses and ball lenses with pin holes are expensive compared to a simple ball lens. For example, a simple ball lens currently costs about $1.00, whereas an aspheric lens currently costs about $4.00 to $5.00. Thus, from a cost standpoint, it would be desirable to be able to use a simple ball lens in a TOSA. A simple ball lens, however, is generally unsuitable for this purpose due to the fact that it produces spherical aberrations that excite the cladding modes of the SM fiber stub 5. Consequently, the only suitable alternative to using the simple ball lens is to use one of the aforementioned more expensive lens configurations.
Accordingly, a need exists for a solution to the problem of modal dispersion that does not require the use of an expensive aspheric lens or a ball lens and pin-hole configuration.